I have a NAD M51 and it is a great sounding DAC with to me what seems like a purely transparent volume control. You can not tell the difference between variable and fixed outputs at normal settings. It is the best digital volume I have experienced and I've owned a few preamps and another DAC with digital volume.
I don't use it as a preamp but my preamp doesn't have remote so I use the M51's volume often. As a source component it is pretty close to perfect for my needs, I have my TV, music server, and BluRay connected to it. The HDMI input is great, I play a lot of music/concert BluRays in 2.1 and the HDMI input will accept the 24/96 content from discs and the 24/192 content from the hard drive I have connected to my BluRay.
It sounds incredible with anything you send to it , even free Pandora at 128k sounds pretty good.
The only con and in my opinion a serious omission, is the lack of front panel volume control.
Other than that I love this DAC
